It is because of heavy de-noising.
The cloudy wall and blurred textures but not edges of some objects give it away.
Turn off denoising and render only portion of your frame (CtrlB to specify region) with more samples (for example 5000 samples is not a lot for final quality). This way you get a 100% size preview on a area you choose quickly. It should be without any blur. You can then lower samples and turn denoising on and see how far from the "ground truth" you deviated.
By tuning the parameters only on a small region area, you can quickly find what the best setting are, it is different scene from scene. You should pick the most difficult area to render, one where you expect the most artifacts.